When it comes to being mobile, having the right accessories certainly makes the task much more productive and enjoyable.

Here is a list of my most crucial mobile accessories:

1. a good phone that you can check your email and browse the web on. I currently use the Apple iPhone and it has become my "first go to device" when mobile
2. a power inverter - I keep one in my car at all times to charge stuff like my tablet pc and MacBook
3. a USB auto power adapter - these little adapters have become some of the most useful accessories I've had in a while. I can keep my phone charged, my wife's Zune, a GPS, and more just by swapping out the USB cable
4. extended batteries - Whenever I buy a new Tablet PC, I almost always get an extended battery for those times that I'm away from power supply
5. extra AC adapter - I always buy at least two AC adapters - one for home and one for the bag. Nowadays, many docking stations are not coming with power adapters, so you have to buy one for the docking station, too
6. a mix of bags - I like to keep a big and small bag handy in my office. If I'm going out of town, I'll likely bring the big bag with me because I have to carry so much of my office with me. When I'm just bopping around town, going to client sites, etc, I'll carry a much smaller bag like Tom Bihn's Ristretto or Buzz Bag
7. In my car, I have a toiletries bag that contains a bunch of miscellaneous items that won't fit in my glove compartment. This small carrying bag is perfect for helping keep my car clean (which is always a struggle for me). I keep extra mini USB to USB cables in there, the inverter, a Zune cable, an iPhone USB cable,  a small screw driver set, and other items I can't remember right now.
8. I always carry a bevy of extra cables in my bag that I store in Tom Bihn's organizer pouches. These include mini USB to USB cables, charging stuff, spare AA batteries, SD cards, USB sticks, etc
9. As noted above, I always carry an assortment of SD cards, USB sticks, and various adapters.
10. A travel power strip - I recently purchased one of these Belkin Mini Power Strips, and absolutely love it. It combines USB and AC into one device. Perfect for the hotel and for leaving those extra USB power adapters at home. These Monster Outlet solutions are also really good. I know Warner uses them quite frequently.
11. When going to conferences and such, I'll carry a small camera with me. However, I mostly just rely on my iPhone's camera to most of my picture taking while mobile.
12. A WWAN solution - I currently use a Novatel USB727 EVDO modem through Verizon
13. A good mounting solution - I've come to really appreciate the mounting solutions available through ProClipUSA.com . I used to consider stuff like that fluff, but now consider them essential accessories. Having the right mounting solution for your phone, Tablet PC ( like the OQO ), or PDA is very helpful in keeping the mess of wires straight, and for keeping your eyes on the road while answering a call, playing music, etc. They help to provide quick touch access to your most used devices. I can't recommend them enough. There are a ton of mounting options out there for most Tablet PCs, too.
14. A really good coffee mug - I've dropped more phones in open coffee cups that sit the center console of my car than I can shake a stick at. I'm currently enjoying a metal thermal mug from Starbucks that sits perfectly in the center cup holder. Nothing beats a great coffee mug for keeping that coffee warm while heading around town.
15. A Molekine journal and pen. I still love writing with pen and paper, and often times, breaking out the Moleskine is the most productive thing I can do when mobile.

There are certainly a lot of other mobile accessories I carry every now and then, but those are the crucial ones - the ones I rely on the most.

What are your "crucial mobile accessories"